In an outboard engine equipped with a two-stroke engine unit, by selecting the exhaust system, especially the capacity and shape of the exhaust expansion chamber thereof or the length of an exhaust pipe protruding into the exhaust expansion chamber, the reflected pressure waves in the exhaust system are exerted upon the exhaust ports so that the engine output and the fuel economy may be improved. In this arrangement, the relationships between the exhaust timing, and the timing at which the reflected pressure waves reach the engine exhaust ports, are substantially determined by the geometrical shape of the exhaust system and by the temperature of the exhaust gases, whereas the intervals between the exhaust gas scavenging timings are varied with variations in the engine speed. Generally speaking, therefore, it is the common practice to design the exhaust system so that the best results are obtained in the vicinity of the normal rpm of the engine.
Since a high rpm range, e.g., at about 5500 rpm is normally used in the outboard engine, the exhaust system is so designed that the best performance can be obtained in such high rpm range. As a result, the output torque in an intermediate rpm range is reduced and a depressed portion is formed in an output torque curve. This undesirably causes deterioration in acceleration, but no counter-measure for effectively solving that problem has been conceived according to the prior art.
The present invention therefore contemplates improvements for such an exhaust device for an outboard engine, which can increase the output torque in an intermediate rpm range without deteriorating the high speed performance it also contemplates doing so in a minimal volume, which is important to an outboard engine installation where space is limited.